On follow up of the comment at: secure ssh...
To disallow direct root login via SSH, edit the "/etc/ssh/sshd_config" file with a text editor and find the following line:
Change the yes to no and remove the comment character at the beginning of the line:
Restart the sshd service.
# service sshd restart
It is also recommended to restrict access to your system by limiting users root access with the su command.
Add trusted users to the special administrative group called wheel via:
# usermod -G wheel <username>
Next open the PAM configuration file for su, "/etc/pam.d/su" in a text editor and remove the comment [#] from the following line:
auth required /lib/security/pam_wheel.so use_uid
The root user is part of the wheel group by default and doing this will permit only members of the administrative group wheel to use the program.
Additionally, you can change the permission on the 'su' binary as below:
# chgrp wheel /bin/su
# chmod 4750 /bin/su
Related Reading: Limit SSH users with PAM
Reference: Linux Security Guide.